1. Field of the Invention
The invention relates to a method of manufacturing a thermal printer head and, more particularly, to a method of forming a heat accumulating layer supporting a heat generating member.
2. Description of the Related Art
Thermal recording systems have been widely used for many years despite the fact that they are not systems for normal paper printing. This wide and continued usage is due, in part, to the recording apparatuses of these systems being compact, easy to maintain, and comparatively inexpensive. A thermal head or a thermal printer head constitutes the major recording section of a thermal recording system. Improvements in the quality of the thermal head or thermal printer have also contributed to the popularity of thermal recording systems. More specifically, in a conventional thermal recording system, characters and images of low image quality are mainly recorded. In contrast to this, with the improvement in resolution, heat response, temperature uniformity and the like of the thermal head, an image of a higher image quality can be obtained at a resolution of 8 dots/mm or higher and at a higher speed. Also, along with the development in the thermal recording method, an image having a quality close to that of a color photograph can be reproduced from an electric image signal obtained from a television receiver, a video set, an electronic camera, and the like.
Generally, methods of manufacturing a thermal head are divided between a thin film method and a thick film method. Although both of them have advantages and disadvantages, the thick film method is generally regarded to be easy since: (1) facility cost is small; (2) productivity is high; (3) it enables manufacture of a large substrate; (4) its patterning process is simple; and (5) materials are used effectively without loss.
As described, for example, in Japanese Patent Application (OPI) Nos. 61-290068 and 61-74865, a general method to form a heat assimilating layer of a thermal printer head in accordance with the thick film method is as follows:
(1) An inorganic powder material which will constitute the heat accumulating layer is formed into a paste; (2) the paste is printed on a substrate a plurality of times using a screen mask having a predetermined pattern; and (3) the resultant substrate is fired at a predetermined temperature.
However, apart from the characteristics required of the material, various drawbacks are posed by screen printing which is seemingly simple.
The heat accumulating layer must have the following characteristics:
(1) When thermal printing is performed, the respective heat generating members must be in contact with the surface of a recording sheet at a uniform pressure. Therefore, the surface of the heat accumulating layer must be quite flat and smooth.
(2) The heat accumulating layer must have sufficient heat resistance and durability to withstand instantaneous temperature increases and decreases during thermal printing. In addition, the heat accumulating layer must have a uniform heat accumulating performance and heat conduction performance. Therefore, no structural defect such as a void or crack may be present in the heat accumulating layer, and the heat accumulating layer must have a uniform, dense structure completely free from surface pinholes.
Regarding these respects, when screen printing is used, screen mesh lines tend to remain because of the printing mechanism. Since a paste is printed through the fine openings of the screen mesh, the paste inevitably tends to form bubbles and generate pinholes. Therefore, the screen printing method is not the most suitable process for forming a heat accumulating layer. In practice, in order to overcome defects such as surface pinholes and to provide a layer having a predetermined thickness, printing and firing are repeated about 2 to 5 times with a paste having a low viscosity, thus resulting in a cumbersome process.